FIG. 1 shows a typical power supply arrangement 100 for a processor 101. As observed in FIG. 1, a power supply unit 105 and a voltage regulator 102 act together to provide the specific supply voltage, with adequate supply current, to the processor 101 over the course of the processor's operation. The voltage regulator 102 provides the specific supply voltage to the processor at processor supply node 103. Modern day processors typically accept a variable range of supply voltages (e.g., 0.6 to 1.8 volts (V)) under the control of the processor itself (for simplicity, the connection from the processor to the voltage regulator 102 or other component to effect supply voltage control is not shown).
In order to provide a “stable” supply voltage to the processor 101, the voltage regulator 102 receives, at input 104, an input voltage that is higher than the supply voltage at supply node 103. For example, modern day voltage regulators that supply a +1.8 V supply voltage can typically accept a voltage anywhere within a range of +4.0 V to +36.0 V at input 104. The voltage regulator 102 therefore “steps down” the voltage received at input 104 (e.g., +12.0 V) to the supply voltage provided at supply node 103 (e.g., +1.8 V). According to one view, the stepping down activity of the voltage regulator 102 permits for a “steady” supply voltage at node 103 in the face of dramatic swings in current draw from the processor 101.
When the processor does draw significant amounts of current, an effect can be observed at input node 104. Specifically, a sudden current draw derived from the increase in power demanded by the processor 101 and the inefficiency of the voltage regulator 102 will be observed at node 104. For example, consider a processor that receives a supply voltage of +1.8 V at supply node 103 and nominally draws a current of 36 Amps (A). A +1.8 V supply voltage and 36 A current draw corresponds to 65 Watts (W) of power dissipation in the processor ((1.8 V)*(36 A)=65 W). The power supply unit 105 will need to supply not only enough power for the processor (65 W) but also additional power to compensate for the less than perfect efficiency of the voltage regulator 102.
For example, if the regulator 102 is 80% efficient, which is presently typical, an additional 20% power increase needs to be provided to the voltage regulator 102 from the power supply unit 105. That is, ((65 W)/0.8)=80 W needs to be provided by the power supply unit 105 to the voltage regulator 102. If the power supply unit 105 feeds a +12 V input voltage to the voltage regulator 102 at node 104, the voltage regulator's current draw from the power supply unit will be ((80 W)/12 V)=6.67 A. (Note that the effect of the step down conversion from +12 V to +1.8 V by the voltage regulator 102 includes comparatively lower current draw demanded by the voltage regulator 102 than the processor 101).
If the processor 101 suddenly increases its current draw demand from 36 A to 56 A, the power supply unit 105 will observe a current draw increase by the voltage regulator 102 from 6.67 A to 10.42 A (assuming the voltage provided by the power supply unit stays fixed at +12 V). That is, the power dissipation in the processor 101 will increase to (56 A)*(1.8 V)=100 W. To account for the less than perfect efficiency of the voltage regulator 102, the power supply unit will need to supply 100 W/0.8=125 W to the voltage regulator 102. Supplying 125 W at +12 V corresponds to 125 W/12 V=10.42 A.
The above analysis bears out that the power supply unit 105, owing to the inefficiency of the voltage regulator 102, is typically designed to supply significantly more power than the processor consumes. Typically, the more power a power supply unit 105 is designed to provide, the larger and more expensive the power supply unit becomes.